The inability to experience pleasure is a key symptom of depression and previous studies have shown that functioning of the nucleus accumbens, a brain region of the size of a hazelnut associated with reward and motivation that is implicated in processing pleasurable stimuli, is impaired in depressed individuals.

During the study, researchers administered DBS treatment in ten patients with severe long-term depression who had not responded to multiple other antidepressant treatments, including psychotherapy, drug treatments and electroconvulsive treatment.

After one year of DBS, all patients showed some improvement, and half of them experienced significant improvement in their symptoms of depression, astonishing considering they had not responded to any prior antidepressant treatment. In addition, the patients showed reduced ratings of anxiety and had only minor side effects. Importantly, none of their overall brain functioning was impaired by the DBS treatment.

"The nucleus accumbens is a brain region that animals will seek to stimulate even if they do not appear depressed and this is one reason that it is sometimes referred to as a reward centre," said Dr. John Krystal, Editor of Biological Psychiatry.

"It is interesting to note that the patients in this study did not simply feel stimulated or euphoric; instead, there appeared to be reductions in depressed mood that paralleled an increase in the capacity for pleasure.

"This finding will stimulate further study on the role of the nucleus accumbens in depression and its treatment," he added.